Allow Private Pages to be parents of other pages

allow-private-parents.php

<?php
/*
Plugin Name: Allow Private Parents
Author: Otto
Author URI: http://ottopress.com
*/

add_filter('page_attributes_dropdown_pages_args', 'allow_private_page_parents');
function allow_private_page_parents($args) {
  $args['post_status'] = array( 'publish', 'private' );
	return $args;
}